Direct injection (DI) engines may produce more soot than port fuel injected engines due to, for example, diffuse flame propagation. As a consequence of diffuse flame propagation, fuel may not adequately mix with air prior to combustion resulting in pockets of rich combustion that generate soot. Further, DI engines may be susceptible to generating soot during high load and/or high speed conditions when there is a lack of sufficient air and fuel mixing.
The inventors herein have recognized various issues in applying particulate filters to DI, spark-ignition engine, for example in maintaining accurate emission control during filter regeneration.
Thus, in one example, a method of controlling an engine having an exhaust system which includes a particulate filter is disclosed. The method comprises, after shutting down the engine and spinning down the engine to rest, operating a vacuum pump to draw fresh air through the exhaust system to an intake system, and regenerating at least a portion of the particulate filter during the engine rest.
In this manner, at least some filter regeneration in a spark-ignited engine may be carried out while the engine is shutdown, thus reducing an impact on engine running emissions. Thus, in the example where filter regeneration also occurs during engine operation, less engine-running filter regeneration may be used.
In one specific example, an exhaust gas recirculation (EGR) system may be employed to facilitate regeneration of the particulate filter. After the engine is shutdown, an EGR valve may be opened and the vacuum pump turned on so that fresh air is drawn through the particulate filter and into the intake manifold via the EGR system. In this way, fresh air containing oxygen (e.g., O2) may be drawn through the particulate filter thus assisting in regeneration of the particulate filter. In another embodiment, positive valve overlap of at least one cylinder being present at the stopped position may be used instead of, or in addition to, the EGR system, to enable fresh airflow from the exhaust system to the intake system while the engine is at rest.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.